(a) Field of the Invention
The present invention relates to liquid crystal displays (LCDs) having storage capacitors, manufacturing methods and a driving method thereof.
(b) Description of the Related Art
In general, a thin film transistor liquid crystal display (TFT-LCD) includes data lines transmitting display signals, gate lines transmitting scan signals, thin film transistors (TFTs) as switching devices, liquid crystal capacitors and storage capacitors. The TFT-LCDs are classified into two modes according to structures of the storage capacitors. One mode has a separate storage line connected to the storage electrodes and the other mode connects the storage capacitor to a gate line.
The principle of driving the LCD in the former mode and a conventional structure of an LCD will be described.
FIG. 1 is an equivalent circuit diagram of a conventional LCD.
A plurality of gate lines G1 and G2 and a plurality of data lines D1, D2 and D3 extend respectively in the horizontal direction and in the vertical direction. The gate lines G1 and G2 and the data lines D1, D2 and D3 intersect each other to define a plurality of pixels. A storage wire COM1 and COM2 passes through the pixels and a TFT is formed in each pixel. A gate electrode (g) of the TFT is connected to the gate line G1 or G2, and a source and a drain electrodes (s and d) of the TFT are respectively connected to the data line and a liquid crystal capacitor (LC). The drain electrode (d) is connected to the storage wire COM 1 or COM 2 to form a storage capacitor (STG).
If a gate-on voltage is applied to the gate electrode (g) of the TFT through the gate line G1, a display signal voltage from the data line is transmitted to the pixels via the TFT and charged in the liquid crystal capacitor (LC) and the storage capacitor (STG). The charged voltage is maintained until the next gate-on voltage in the next frame is applied in the pixel. Generally, when the gate voltage is changed from xe2x80x9conxe2x80x9d level to xe2x80x9coffxe2x80x9d level, the pixel voltage slightly drops. The storage capacitor reduces the voltage-drop.
The TFTs have amorphous silicon layers or polycrystalline silicon layers as active layers and are classified into a top gate mode and a bottom gate mode according to the relative location of the gate electrode and the active layer. Most of the polycrystalline silicon TFT-LCDs use the top gate mode.
A storage capacitor of the conventional polycrystalline silicon TFT-LCD includes a doped storage region in the silicon layer, a storage electrode overlapping the storage region and a gate insulating film interposed therebetween. Moreover, another storage capacitor is formed of the storage electrode, a pixel electrode overlapping the storage electrode and a dielectric including an interlayer insulating film and a passivation film interposed between the pixel electrode and the storage electrode. However, the capacitance between the pixel electrode and the storage electrode is relatively small and negligible, since the interlayer insulating film and the passivation film, which is respectively 5,000 xc3x85 thick, are much thicker than the gate insulating film, which is 500 xc3x85xcx9c3,000 xc3x85 thick.
In this conventional structure, it is required to add an ion implanting step to make a storage region. In other words, additional steps of depositing photoresist film, patterning the photoresist to make openings by using a mask, injecting ions into the silicon layer through the openings and annealing the injected ions are required.
It is therefore an object of the present invention to reduce the manufacturing steps by getting rid of photolithography and ion implanting in the steps of forming a TFT and a storage capacitor.
It is another object of the present invention to obtain a sufficient storage capacitance.
It is another object of the present invention to reduce the storage capacitance difference between pixels.
It is another object of the present invention to reduce the effective resistance in a storage region which serves as one of the electrodes of a storage capacitor.
To solve these objects, a silicon layer of an LCD according to the present invention includes a doped source region, a doped drain region, an undoped channel region and an undoped storage capacitor region. The channel region is located between the source and the drain region, and the storage capacitor region is next to the drain region. A gate insulating film is formed on the silicon layer, a gate electrode is formed on the gate insulating film opposite the channel region, and a storage electrode is formed opposite the storage capacitor region. In other words, a storage capacitor includes the storage capacitor region, the storage electrode and the gate insulating film interposed therebetween. Since the storage capacitor region is not doped, the storage capacitor can be used by applying to the storage electrode a voltage which is equal to or larger than the sum of the threshold voltage of the TFT and the maximum value of the display signals voltage.
This LCD is manufactured by forming a silicon layer and a gate insulating film, forming a gate electrode and a storage electrode, and implanting ions into the silicon layer using the gate electrode and the storage electrode as a mask.
To solve the above-mentioned object, in another LCD according to the present invention, a metal pattern for a source electrode and a metal pattern for a storage electrode are formed, and a silicon layer is formed on the two patterns. Regions of the silicon layer which contact the patterns are doped to become a source and a drain regions. A gate insulating film is formed on the silicon layer and the metal pattern for a storage electrode, and a storage electrode is formed on the gate insulating film opposite the metal pattern for a storage electrode. In other words, a storage capacitor includes the metal pattern for a storage electrode, the storage electrode, and the gate insulating film interposed therebetween.
Since a passivation film is formed on the storage electrode, and a pixel electrode is formed on the passivation film opposite the storage electrode, another storage capacitor that includes the pixel electrode, the storage electrode and the passivation film may be formed.
The pixel electrode may contact the drain region of the silicon layer, and the metal pattern for a storage electrode.
In this manufacturing method, the step of forming contact holes to connect a silicon layer to a metal pattern for storage electrode is omitted by forming a metal pattern for storage electrode of the metal for data wire and then forming a silicon layer on the metal pattern for a storage electrode.
To solve the above-mentioned object, in another LCD according to the present invention, a gate electrode and a storage electrode are formed on a gate insulating film which is formed on an insulating substrate, and an interlayer insulating film covers the gate electrode and the storage electrode. A passivation film is formed on the interlayer insulating film. Since a certain amount of the passivation film and the interlayer insulating film over the storage electrode is removed, the thickness of dielectric of the storage capacitor can be thinner, which means that the capacitance can be increased.
A double-layered structure or a multi-layered structure may be adopted to achieve a uniform thickness for the interlayer insulating film. In this case, the most upper layer is made of a material having a similar etch rate to the passivation film and the lower layers are made of materials having a lower etch rate than the most upper layer so that the most upper layer can be removed and the other layers can remain in the step of removing the passivation film over the storage electrode.
To solve the above-mentioned object, a silicon layer of an LCD according to the present invention includes doped regions of source and drain, an undoped channel region, a doped storage capacitor region, and sub-regions. The channel region is located between the source region and the drain region, the storage region is adjoining to the drain region and disconnected from the channel region, and the sub-regions are adjoining to edges of the storage capacitor regions and connected to the drain region. A gate insulating film is formed on the silicon layer, a gate electrode and a storage electrode are formed on the gate insulating film opposite the respective channel region and storage capacitor region. In other words, a storage capacitor includes the storage capacitor region, the storage electrode and the gate insulating film interposed therebetween. The storage capacitor region cannot be used as one electrode of a storage capacitor in the xe2x80x9coffxe2x80x9d state because it is not doped. However, in the xe2x80x9conxe2x80x9d state where a voltage which is equal to or larger than the sum of threshold voltage of the TFT and the maximum value of the image signals is applied to the storage electrode, the storage capacitor region can be used as an electrode of a capacitor. In this structure, the sub-regions decrease the resistance of the charge accumulation layer, because it becomes a portion of charge transmitting paths.
This LCD can be manufactured by forming a silicon layer and a gate insulating film, forming a gate electrode and a storage electrode thereon, and then implanting ions into the silicon layer using the gate electrode and the storage electrode as a mask.
To solve the above-mentioned object, in another LCD of the present invention, a silicon layer is formed on an insulating substrate and a gate insulating film covers the silicon layer including a doped source and drain regions, and an undoped channel region interposed between the source and the drain regions. A gate electrode is formed on the gate insulating film opposite the channel region, and a storage capacitor including a lower storage capacitor electrode made of a metal for gate wire, an insulating film on the storage capacitor electrode and an upper storage capacitor electrode on the insulating film is formed. The upper storage capacitor electrode is connected to a transparent pixel electrode.
It is required to form the upper and the lower storage capacitor electrodes and the insulating film having the same pattern.
Moreover, each of the upper and the lower storage capacitor electrodes and the insulating film interposed therebetween may be double-layered or multi-layered.
In the manufacturing method of the above LCD, the storage capacitor is manufactured by sequentially depositing a metal film for a gate wire, an insulting film for a storage capacitor, and a metal film for a storage capacitor, patterning the three films to form a storage capacitor and a gate wire including a gate electrode, implanting ions into a silicon layer using the gate electrode as a mask to form a source and a drain regions, and then forming a transparent pixel electrode contacting the metal for the storage capacitor.
An interlayer insulating film and a passivation film may be deposited over the storage capacitor. It is desirable to simultaneously remove a portion of the interlayer insulating film and the passivation film to expose the metal film for storage capacitor by forming the interlayer insulating film and the passivation film as materials having the same etch rate. The metal film for storage capacitor works as an etch-stopper in the etching step.
As described above, since the storage capacitor is formed by the lower and the upper storage capacitor electrodes and an insulating film with a thin thickness, it is possible to obtain a sufficient storage capacitance. Since the thickness of the insulating film in the pixels is uniform, it is possible to reduce a storage capacitance difference between the pixels. Moreover, since it is not required to inject ions into the silicon layer to form a storage electrode, the manufacturing process becomes simple.